Plant breeding is one of the fundamental occupations of mankind that is of pivotal importance for providing domesticated species that feed the world. Plant breeding is by consequence old and was originally based on selecting and propagating those plants that were outperforming in local selection fields.
Contemporary plant breeding is highly depending on knowledge of genetics and technologically supported by methods such as doubled haploids (DH) (see fi. Haploids in Crop Improvement II eds; Palmer C, Keller W, and Kasha K (2005) in: Biotechnology in Agriculture and Forestry 56 Eds; Nagata T, Lörz H, and Widholm J. Springer-Verlag Berlin Heidelberg N.Y., ISBN 3-500-22224-3) and molecular markers (see fi. De Vienne ed. (2003) Molecular Markers in Plant Genetics and Biotechnology. Science publishers Inc. Enfield, N.H. USA. ISBN 1-57808-239-0).
Genetic mechanisms during sexual reproduction have evolved to increase genetic variation which enhances the chances of survival of a species in a changing environment. Meiotic recombination, independent chromosome assortment and the mating system are main contributing factors in this respect. However, in plant breeding these mechanisms may act counterproductive especially in those cases when genetically heterozygous plants have been identified with high agronomic or horticultural value. Redistribution of genetic factors results in the generation of genetically dissimilar and therefore heterogeneous plants and by consequence loss of commercially desirable traits.
In order to counter this effect, a number of technologies is available to the plant breeder. One possibility is to propagate plants vegetatively which leads to a complete preservation of their genetic composition as multiplication occurs exclusively through mitosis. For many plant species, in vitro tissue culture is being used to vegetatively propagate plants although other methods like producing cuttings in vivo may be applicable as well.
A disadvantage of vegetative propagation when compared to propagation through seeds is the fact that it is labour intensive and thereby costly. Furthermore, it is difficult to store plants for longer periods of time posing logistic problems and the risks of infections of the plant material with pathogens, like viruses, is considerably larger as compared to a situation in which plant material is propagated through seeds.
As an alternative, vegetative propagation may be achieved through the formation of asexual seeds, which is generally referred to as apomixis. Apomixis which occurs naturally in a number of species may be induced in sexually propagating plant species by genetic engineering. Currently, however, the genes responsible for the different steps of apomixis i.e. apomeiosis, parthenogenesis and autonomous endosperm development have not yet been identified and may interact in a complicated manner. Therefore, although the potential of apomixis technology for plant breeding is widely recognised for already a long period of time, proof of concept is still awaited.
As yet another alternative, use can be made of reverse breeding technology as described in WO-03017753. Reverse breeding is based on the suppression of meiotic recombination through genetic engineering and the subsequent production of doubled haploid plants (DHs) derived from spores containing unrecombined parental chromosomes. These DHs differ with respect to their genetic composition solely as a consequence of the independent parental chromosome assortment which occurred during meiosis. Therefore, it is sufficient to make use of one co-dominant, polymorphic marker per chromosome to determine which of the DHs or lines derived therefrom should be combined through crossing to reconstruct the genetic composition of the original starting plant. As such, application of reverse breeding technology allows genetic preservation of any selected fertile plant through seeds even if its genetic composition is unknown.
However, a disadvantage of this technology is the fact that complete suppression of meiotic recombination results in the absence of chiasmata and thereby inappropriate chromosome segregation during meiosis I which could lead to aneuploidy of the gametes and thereby reduced viability. When no chiasmata are formed during meiosis I, every chromosome has an independent 50% chance to move to either one of the poles. This means that the theoretical chance to make a spore with a full chromosome complement is (½)x wherein x represents the haploid chromosome number. The frequency of balanced gametes therefore decreases with increasing haploid chromosome number.
Although many crop species have a relatively low chromosome number (e.g. cucumber has 7 chromosomes per haploid genome and spinach has only 6) there are also economically important species with relatively high chromosome numbers like tomato, one of the largest vegetable crops, which has 12 chromosomes per haploid genome. This technical constraint significantly reduces the efficiency of reverse breeding technology. Therefore, a clear need in the art exists for alternative methods which allow preservation of genetic composition in sexual offspring.
It is therefore the object of the present invention to provide a method for preserving the genetic composition of a parent organism, in particular a parent plant.